1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
U.S. Pat. Nos. 5,339,737 and 5,379,698, the entire disclosures of which are hereby incorporated by reference, disclose a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge (see, e.g., U.S. Pat. No. 5,385,092 and U.S. application Ser. No. 08/376,766). These include "wet" plates that utilize fountain solution during printing, and "dry" plates to which ink is applied directly.
In particular, the '698 patent discloses laser-imageable plates that utilize thin-metal ablation layers which, when exposed to an imaging pulse, decompose into gases and volatile fragments even at relatively low power levels. The remaining layers are solid and durable, generally of polymeric or thicker metal composition, enabling the plates to withstand the rigors of commercial printing and exhibit adequate useful lifespans.
In one general embodiment, the plate construction includes a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid. Underlying the first layer is a thin metal layer, which ablates in response to imaging (e.g., infrared, or "IR") radiation. A strong, durable substrate underlies the metal layer, and is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the absorbing second layer by an imaging pulse weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step. This, once again, creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer.
In this type of construction, imaged areas are easily distinguished from unimaged areas. The substrate is typically clear, so that the silvery appearance of regions that have not received laser exposure ordinarly contrasts with the surface (e.g., a plate cylinder or inspection table) underlying the printing member. This is not, however, the case with other types of constructions.
For example, as outlined in the '737 patent and U.S. Pat. No. 5,570,636 entitled LASER-IMAGEABLE LITHOGRAPHIC PRINTING MEMBERS WITH DIMENSIONALLY STABLE BASE SUPPORTS (the entire disclosure of which is hereby incorporated by reference), it is possible to laminate the above-described construction to a metal support that not only provides dimensional stability, but also acts to reflect unabsorbed imaging radiation back into the thin metal layer. Assuming clear substrate and laminating adhesive materials, however, the metal support is likely to offer little contrast to the thin-metal imaging layer.
Also as described in the '636 patent, it is possible to utilize thin-metal imaging layers over metal base supports without lamination. Although thermally conductive metal supports would dissipate imaging energy if disposed directly beneath the thin metal layer, the '636 patent details constructions that concentrate heat in the thin metal layer, preventing (or at least retarding) its transmission and loss into the base support. To accomplish this, a thermally insulating layer is interposed between the imaging layer and the thermally conductive base support. Once again, assuming that the insulating layer is fabricated from a clear polymeric material, contrast between the thin metal layer and the metal base support will be minimal.
Printers have traditionally exploited contrast between imaged and unimaged plate regions to facilitate visual inspection. Typically, the press operator first utilizes the gross patterns to ensure that the plate corresponds to the current job, and that the series of plates on successive plate cylinders correspond to one another. He can then inspect the contrasting regions of the plates more closely, verifying proper overall imaging and the presence of key details prior to operating the press. The absence of contrast makes it difficult or impossible for a press operator to perform these identification and inspection activities by examination of the plate. Although the press operator can prepare a proof to obtain direct visualization of the plate image, this is time-consuming operation, particularly in a computer-to-plate environment.